How to connect a noise free system in three easy steps. OK maybe four...
- Use cable TV ground isolator . This is the single biggest source of noise.
- Ground your receiver/ preamp This power strip can use cable TV coax to connect your receiver to it's grounded outlet. It also has 12V trigger controlled outlets. Panamax M8-AV-Pro. An inexpensive alternative is to run a piece of hookup wire from the receiver's antennae or phono ground to the cover plate screw on the AC outlet.
- Use only RCA to XLR adapter cables with shielded twisted pair construction like these from Markertek and Monoprice. Shield, Ground (pin1), and Pin 3 (white or minus) should all be connected together at the RCA (sending) end only. The RCA tip connects to pin 2 in the XLR.
- If XLR to XLR connection is used and you still have hum, chances are your preamp has a 2 wire AC cord and the power amp has a 3 wire AC cord. In this case, ground the preamp as in Step 2 above, AND lift the ground wire inside the XLR at the amp end by cutting the wire to pin 1 inside the shell or using an XLR ground lift adapter like this. If the shell of the XLR is connected to ground, you may have to remove this connection as well.
- Add an input transformer. Like this one for XLR Jensen ISOMAX GX or for RCA Jensen SUB-1RR. or Blue Jeans Cable Isolation Transformer. Maybe try a cheap one to see if it fixes the problem, then buy the nice Jensen one for the low distortion and wide frequency response. https://www.amazon.com/Transformer-Eliminator-Blue-Jeans-Cable
- Turn the amp off when preamp is off. Some systems will have noise if the preamp/receiver is turned off but the amps are left on. Xantech AC1 allows 12V trigger equipped preamps to turn off the power to the amp. At $25 hard to beat this one from IoTPower Relay. Or this load sensing power strip can also turn off the amps Smart Strip SCG3 Energy Saving Power Strip with Autoswitching Technology. Or these wireless remote controlled outlets Woods 13569 Indoor Wireless Remote Control (3-Pack). Or just leave the preamp on all the time....
- When it is time to call the electrician. Sometimes an installation is still too noisy even when everything is done right as above. It is possible that the AC wiring was done wrong and must be fixed by a professional. Have the electrician very carefully check the AC power circuit of the lights. Current is traveling around a physically large loop. The hot and neutral must stay together in the same conduit, ideally twisted together, if they travel separated routes, they become a magnetic field generator. The most likely culprits are improperly routed 3-way switch, improperly routed Edison shared neutral circuits, ground-to-neutral bond out on the branch that allows the return current to travel the wrong route, or a batch of neutrals from several circuits bundled together. All of these are dangerous safety violations that must be fixed immediately. The most common of these is an extra ground-to-neutral bond. According to the National Electrical Code, there should be one and only one connection ("bond") between the neutral and ground and that should be inside the main breaker panel at the service entrance (where the wire from the street comes into the building). Unfortunately, your typical breaker box ("sub panel") from Home Depot has this connection installed as a default and many (most?) electricians do not know to remove the bond when installing it as a subpanel! If these subpanels are installed distant from the main panel with N-G bonds inside them, you have a noise field generator. These connections inside the subpanels must be removed. And there may be more than one subpanel.